Bottom Line:
The opposite was found for pair-housed animals.Single-housing increased the time spent on stretch attend postures.However, pair-housing reverses these effects possibly due to stress from dominance disputes between pairs.

Affiliation: Queens Medical Centre, School of Life Sciences, University of Nottingham, Nottingham, NG7 2UH, United Kingdom.

ABSTRACT

Background: Lack of physical activity and increased levels of stress contribute to the development of multiple physical and mental disorders. An increasing number of studies relate voluntary exercise with greater resilience to psychological stress, a process that is highly regulated by the hypothalamic-pituitary-adrenal (HPA) axis. However, the molecular mechanisms underlying the beneficial effects of exercise on stress resilience are still poorly understood. Here we have studied the impact of long term exercise and housing conditions on: a) hippocampal expression of glucocorticoid receptor (Nr3c1), b) epigenetic regulation of Nr3c1 (DNA methylation at the Nr3c1-1F promoter and miR-124 expression), c) anxiety (elevated plus maze, EPM), and d) adrenal gland weight and adrenocorticotropic hormone receptor (Mc2r) expression.

Results: Exercise increased Nr3c1 and Nr3c1-1F expression and decreased miR-124 levels in the hippocampus in single-housed mice, suggesting enhanced resilience to stress. The opposite was found for pair-housed animals. Bisulfite sequencing showed virtually no DNA methylation in the Nr3c1-1F promoter region. Single-housing increased the time spent on stretch attend postures. Exercise decreased the time spent at the open arms of the EPM, however, the mobility of the exercise groups was significantly lower. Exercise had opposite effects on the adrenal gland weight of single and pair-housed mice, while it had no effect on adrenal Mc2r expression.

Conclusions: These results suggest that exercise exerts a positive impact on stress resilience in single-housed mice that could be mediated by decreasing miR-124 and increasing Nr3c1 expression in the hippocampus. However, pair-housing reverses these effects possibly due to stress from dominance disputes between pairs.

Mentions:
Another epigenetic mechanism that can regulate Nr3c1 levels is silencing by non-coding RNAs. miR-124, a highly conserved microRNA (Fig. 4a) that is particularly enriched in the brain, was shown to repress Nr3c1 expression both in vitro and in vivo [22]. Thus we studied whether exercise and/or housing conditions affected miR-124 expression in the hippocampus. We found that the effects of exercise and housing on miR-124 expression follow a pattern opposite to the one for Nr3c1. Pair-housed animals expressed lower levels of miR-124 [Two-way ANOVA, F (1,25) = 4.604 p < 0.042] (Fig. 4b). Moreover there was a significant housing*exercise interaction [F (1,25) = 6.302 p = 0.019]. Post hoc comparisons indicated that sedentary single-housed mice had significantly higher miR-124 expression than mice that exercised [t (14) = 2.485 p = 0.026] (Fig. 4b) and sedentary pair-housed mice [t (11) = −3.295 p = 0.007] (Fig. 4b). Additionally, Nr3c1 and miR-124 levels were negatively correlated in the single-housed group [Pearson’s r (15) = −0.525, p = 0.045] but not in the pair-housed group [Pearson’s r (11) = 0.046, p = 0.892]. Since miR-124 has previously been shown to regulate Nr3c1 [22], it could be hypothesized that the negative correlation between Nr3c1 and miR-124 levels is due to a direct effect of decreased miRNA-124 on Nr3c1 expression. However, further experiments are needed in order to validate whether this is a direct causal relationship. Pair-housing reduced both Nr3c1 and miR-124 expression in the hippocampus. This could suggest the involvement of alternative epigenetic mechanisms probably activated by stress linked to pair-housing. A possible mechanism could be decreased histone acetylation in the pair-housed group due to higher stress levels. For example chronic variable stress significantly decreases histone acetylation (H3k12Ac) in the hippocampus [41]. Moreover, treatment with the histone deacetylase (HDAC) inhibitor TSA significantly increased Nr3c1 expression in the hippocampus [18].Fig. 4

Mentions:
Another epigenetic mechanism that can regulate Nr3c1 levels is silencing by non-coding RNAs. miR-124, a highly conserved microRNA (Fig. 4a) that is particularly enriched in the brain, was shown to repress Nr3c1 expression both in vitro and in vivo [22]. Thus we studied whether exercise and/or housing conditions affected miR-124 expression in the hippocampus. We found that the effects of exercise and housing on miR-124 expression follow a pattern opposite to the one for Nr3c1. Pair-housed animals expressed lower levels of miR-124 [Two-way ANOVA, F (1,25) = 4.604 p < 0.042] (Fig. 4b). Moreover there was a significant housing*exercise interaction [F (1,25) = 6.302 p = 0.019]. Post hoc comparisons indicated that sedentary single-housed mice had significantly higher miR-124 expression than mice that exercised [t (14) = 2.485 p = 0.026] (Fig. 4b) and sedentary pair-housed mice [t (11) = −3.295 p = 0.007] (Fig. 4b). Additionally, Nr3c1 and miR-124 levels were negatively correlated in the single-housed group [Pearson’s r (15) = −0.525, p = 0.045] but not in the pair-housed group [Pearson’s r (11) = 0.046, p = 0.892]. Since miR-124 has previously been shown to regulate Nr3c1 [22], it could be hypothesized that the negative correlation between Nr3c1 and miR-124 levels is due to a direct effect of decreased miRNA-124 on Nr3c1 expression. However, further experiments are needed in order to validate whether this is a direct causal relationship. Pair-housing reduced both Nr3c1 and miR-124 expression in the hippocampus. This could suggest the involvement of alternative epigenetic mechanisms probably activated by stress linked to pair-housing. A possible mechanism could be decreased histone acetylation in the pair-housed group due to higher stress levels. For example chronic variable stress significantly decreases histone acetylation (H3k12Ac) in the hippocampus [41]. Moreover, treatment with the histone deacetylase (HDAC) inhibitor TSA significantly increased Nr3c1 expression in the hippocampus [18].Fig. 4

Bottom Line:
The opposite was found for pair-housed animals.Single-housing increased the time spent on stretch attend postures.However, pair-housing reverses these effects possibly due to stress from dominance disputes between pairs.

Affiliation:
Queens Medical Centre, School of Life Sciences, University of Nottingham, Nottingham, NG7 2UH, United Kingdom.

ABSTRACT

Background: Lack of physical activity and increased levels of stress contribute to the development of multiple physical and mental disorders. An increasing number of studies relate voluntary exercise with greater resilience to psychological stress, a process that is highly regulated by the hypothalamic-pituitary-adrenal (HPA) axis. However, the molecular mechanisms underlying the beneficial effects of exercise on stress resilience are still poorly understood. Here we have studied the impact of long term exercise and housing conditions on: a) hippocampal expression of glucocorticoid receptor (Nr3c1), b) epigenetic regulation of Nr3c1 (DNA methylation at the Nr3c1-1F promoter and miR-124 expression), c) anxiety (elevated plus maze, EPM), and d) adrenal gland weight and adrenocorticotropic hormone receptor (Mc2r) expression.

Results: Exercise increased Nr3c1 and Nr3c1-1F expression and decreased miR-124 levels in the hippocampus in single-housed mice, suggesting enhanced resilience to stress. The opposite was found for pair-housed animals. Bisulfite sequencing showed virtually no DNA methylation in the Nr3c1-1F promoter region. Single-housing increased the time spent on stretch attend postures. Exercise decreased the time spent at the open arms of the EPM, however, the mobility of the exercise groups was significantly lower. Exercise had opposite effects on the adrenal gland weight of single and pair-housed mice, while it had no effect on adrenal Mc2r expression.

Conclusions: These results suggest that exercise exerts a positive impact on stress resilience in single-housed mice that could be mediated by decreasing miR-124 and increasing Nr3c1 expression in the hippocampus. However, pair-housing reverses these effects possibly due to stress from dominance disputes between pairs.